The SNES was pioneering with the use of shoulder buttons - F-Zero made great use of them for directional air brakes.

As for the kit, it looks like it'll work with any pad as it's basically wiring directly to the button switches, effectively bypassing whatever internals were already there. You may need some soldering and poking around with a multimeter but I can't see why it wouldn't work for any other pad... Though Megadrive pads are expensive, so there are cheaper options for gutting a pad.